Alert archive / California / Mesoscale Discussions in 2021

Mesoscale Discussion 41

January 27, 2021Alpine County, Tuolumne County, California

The Storm Prediction Center issued this mesoscale discussion at Jan 27, 2021, 3:31 PM PST for Alpine County, Tuolumne County, California. It was in force for 6 hr.

The area

The outline as issued.

The facts

Issued: Jan 27, 2021, 3:31 PM PST (23:31 UTC)
Expired: Jan 27, 2021, 9:30 PM PST
In force: 6 hr
Issued by: the Storm Prediction Center
Concerning: Heavy snow

Counties Alpine County, CA, Tuolumne County, CA

The radar

The San Joaquin Valley radar (KHNX), 169 km from the middle of the area, began a volume at 23:32 UTC, the scan nearest the moment this went out.

Open the radar archive at this moment

The archived warnings, watches and discussions in force at that minute are drawn over the scan, this one among them.

The nearest sounding before it

Reno, NV (KREV), launched 12:00Z, 11.5 hours before the alert went out and 221 km from the middle of its area. This is the routine balloon launch nearest before it, the air the forecaster had in hand, not a measurement taken for the alert.

Skew-T log-p

Temperature in red, dew point in green, the mixed-layer parcel dashed.

Hodograph

The wind turning with height, colored by layer: ground to 1 km, 1 to 3 km, 3 to 6 km, 6 to 10 km.

Mixed-layer CAPE: 0 J/kg
Mixed-layer CIN: No buoyant parcel
Mixed-layer LCL: 620 m above ground
0-6 km bulk shear: 72 kt
0-1 km storm-relative helicity: 107 m²/s²
0-3 km storm-relative helicity: 279 m²/s²
Significant tornado parameter: 0.0
Supercell composite: 0.0

Open this sounding in Storm Lab

The text

As transmitted.

866 
ACUS11 KWNS 272331
SWOMCD
SPC MCD 272331 
CAZ000-280530-

Mesoscale Discussion 0041
NWS Storm Prediction Center Norman OK
0531 PM CST Wed Jan 27 2021

Areas affected...Sierra Nevada

Concerning...Heavy snow 

Valid 272331Z - 280530Z

SUMMARY...Heavy snowfall rates in the 3-5 inches per hour range are
likely this evening for the western slopes of the Sierra,
particularly in areas of 4000-8000 ft elevation.

DISCUSSION...Water-vapor imagery late this afternoon shows a
mid-level trough situated over the eastern Pacific west of the OR
and northern CA coasts.  The low, mid, and high-layer
moisture-sensing GOES imagery bands indicate a deep tropospheric
fetch of substantial moisture optimally oriented from the eastern
Pacific northeastward into the Sierra Nevada.  

The thermally direct circulation of an upper speed max will continue
to be located over the Sierra Nevada through tonight.  Coupled with
the strong large-scale ascent, orographic forcing will result in
strong upward vertical motion on the western slopes of the Sierra. 
The aforementioned Pacific moisture will contribute to a strong
moisture flux in the region as lessening static stability/latent
heating occurs near the western slopes as precipitation rates
increase this evening.  Model guidance indicates snowfall rates on
the western slopes between 4000-8000 ft elevation will range from
3-5 inches per hour.  During some of the heavier precipitation
bursts, appreciable spillover snowfall later this evening may occur
on the lee side near the CA/NV border.

..Smith.. 01/27/2021

...Please see www.spc.noaa.gov for graphic product...

ATTN...WFO...VEF...REV...HNX...STO...

LAT...LON   36731861 36781881 37571955 38532045 38922057 39042034
            38791985 38491962 37581894 36971846 36731861

Nearby

Mesoscale Discussions in California, 2021 Every mesoscale discussion in 2021

Sources

The product and its follow-ups from the Iowa Environmental Mesonet's archive of National Weather Service and Storm Prediction Center text; the sounding from the Mesonet's radiosonde archive; radar from the NEXRAD Level II archive.